today i updated my birt environment from 2.3.0 to 2.3.1 using eclipse
update function on WinXP (Client). Execution of the "test.rptdesign" works.

Then i downloaded the 2.3.1 runtime to deploy it on the jboss webserver
(version 4.2.3.GA) running on RHEL 5.2 64-bit. (Deployment as described
for jboss on the BIRT website http://www.eclipse.org/birt/phoenix/deploy/viewerSetup.php ). (With
version 2.3.0 the "test.rptdesign" (and other reports) are running
without that problem).

I had the same problem today and I solved the problem by deleting the
folder "tmp" and "work" under server/default/deploy/ in the JBoss
application server.

In my case, the .java and .class files, the JBoss Web-Container creates
out of the .jsp files of the WebViewer were not recreated by the
deployment and so still were those of BIRT 2.3.0 instead of 2.3.1, which
lead to the same Exception you are getting.

If you delete the "tmp" and "work" folders, the application should run
without any problems.